SLMs can be transmissive or reflective, based on micromechanical shutters or mirrors, or on reflecting liquid crystals or other electro-optical cells. Much of this invention relates to micromechanical reflecting SLMs, i.e. micromirror arrays. Reflecting micromechanical SLMs have the advantage that they can be inexpensive and stable, and can have high power handling capability and a very high data rate. They can be built in large arrays of many million mirrors and the operating speed is more limited by the data loading speed than the operation of the mirrors themselves, since each mirror may operate in the hundreds of kilohertz or even megahertz range. Were it not for the bottleneck of bringing the data on to the chip a micromirror array could easily operate at 1012 pixel operations per second. Another advantage is that they can be used with light of very short wavelength, for example in deep UV or even extremely UV (soft x-ray) light.
Many different types of spatial light modulators have been disclosed in patents and at conferences. Four different types of micromirror arrays are technically important and do at the same time illustrate variations in design and operating principles among mirror arrays SLMs: the Texas Instruments' DMD mirror arrays mainly used for projection (U.S. Pat. No. 5,583,688); Micronics' tilting mirrors used for lithography (U.S. Pat. No. 7,009,753) 1§0020 including the SLM with a phase step from /Ljungblad et al./; piston-type SLMs for lithography and wavefront correction shown by Lucent and FhG-IPMS; and one-dimensional arrays for projection displays and lithography by Silicon Light Machines (U.S. Pat. No. 5,459,610). When looking at reflecting micromechanical SLMs it is useful to understand the properties of these array types.
Some work by specular reflection, like TI's DMDs (U.S. Pat. No. 5,583,688), and others by diffraction, like most other types. In diffraction, the phase differences within pixels or between pixels are used to modulate the light. In specular reflection the direction of the pixel surface sends the reflected beam into the accepting aperture of the optics, or outside of it. Another distinction between different SLMs may be if the light is coherent between adjacent mirrors or not. When TI's DMDs are used, the light is typically not coherent between mirrors; in piston-type SLMs it must be coherent; and in Silicon Light Machines' devices it may or may not be coherent between pixels depending on the device and the system design. The third distinction is the type of actuation, that is, whether the mirrors are moving up and down like pistons or tilting like swing boards. A fourth distinction is whether the phase of the electric field, the magnitude, or both are modulated when the mirrors are actuated. Finally, the operation may be on-off (“digital”) or have multiple states (“analog”).
Analyzing the SLMs above one finds that TI's DMD design falls in a group by itself: specular, incoherent between mirrors, tilting, modulating only the amount of light through the optics, and on-off. Most other devices work by diffraction, have multiple states and at least some degree of coherence between adjacent mirrors. We may call these two groups incoherent and coherent modulators. Among coherent modulators, i.e. the SLMs from Micronic, Silicon Light Machines, FhG-IPMS and Lucent, the type of modulation and the type of actuation varies.
A common property of diffractive SLMs is that they are monochromatic. They create darkness through destructive interference and the destructive phase relation is perfect at only one wavelength. Many SLMs can be used for different wavelengths by using one wavelength at a time and tuning the (analog) actuation between different wavelengths.
Diffractive, micromirror arrays typically change the phase of the electric field, its magnitude, or a combination of both when they are actuated. This can be described as a trajectory in the complex plane, the trajectory that the reflected E-field phasor traces in the complex plane when the mirror is gradually activated from relaxed to fully activated. Different applications require or are best served by different trajectories and the trajectory of an SLM can be modified by the methods disclosed, in particular, by creation of height steps and other surface profiles on the mirror surface.
Apply surface profiles with a variety of properties, can be built using the same driving circuits, by only modifying the surface profiles of the micro mirrors.
Still another aspect of the invention is that tilting mirrors can be made into fully complex modulators by means of height steps. Fully complex modulation is known in the art (Florence, J. M., “Full Complex Spatial Filtering with a Phase Mostly DMD”, Proc. SPIE 1558, pp. 487-498 (1991); U.S. Pat. No. 7,227,687).
Texas Instruments' DLP micromirror arrays for digital projectors and digital cinema are reflective micromechanical SLMs. The recent availability of TI's devices on OEM basis has spawned a wide range of industrial applications. TI's micromirrors are non-coherent on-off modulators which essentially turn the light on and off at each mirror. These modulators have been applied with considerable ingenuity to a variety of applications. Some applications are best served by a SLM with a coherent illumination source and, therefore, are not well served by the DMD. The success of the DMD in OEM applications shows the power of the SLM as an optical building block, but there are still large application fields waiting for the ideal SLM to emerge. The SLMs disclosed in this application are intended to advance SLM technology towards usefulness in these other application fields.
Coherent spatial light modulators, which preserve the phase and coherence properties of the light from mirror to mirror, have been used for lithography by Micronic Laser. A diffractive micromirror array is used as an image modulator and to project deep UV light on photoresist, creating a pattern with features as small as 0.1 microns.
One difference between non-coherent and coherent modulators is the amount of movement. A coherent or diffractive modulator has mirrors which move by a fraction of the wavelength, while non-coherent modulators have mirror flaps which tilt to send the specular reflection in a different direction. The non-coherent modulators tip tens of wavelengths at their edge. A coherent modulator may form a perfect phase surface, while the phase is less well-controlled or not controlled at all in a non-coherent modulator. Good phase control is important in certain applications like holography and wave front correction.
There are two main types of diffractive micromechanical modulators: piston mirrors and tilting mirrors. A piston mirror moves up and down, changing the phase of the light that is reflected from it, while keeping the magnitude of the reflected light constant. A tilting mirror has the opposite properties: it changes the magnitude of the reflected light while keeping its phase constant. These two types have different applications. The piston type is better for beam steering, wave front correction, and holography. The tilting type on the other hand is better for high quality lithography. However, the applications overlap. The piston type can be used to lithography with a more complex rasterization. Moreover, in some applications neither type is perfect, since what is needed is really a fully complex device which can modulate both magnitudes and phases. For example, holography needs a fully complex modulator, and both tilting and piston modulators are approximations. An important property of fully complex modulators is that they can be used to form diffraction patterns close to the optical axis with good suppression of zero-order (i.e. non-diffracted) light and of mirror (a.k.a. conjugate) and higher-order images.